Near Field Communication (NFC) is one of the wireless communication systems operating in a frequency range around 13 MHz for a short range within 10 cm between electromagnetically-coupled circuits each having an antenna coil. An NFC-compliant circuit can be installed in various media, for example, electronic instruments such as cards, tags, and mobile phones.
Typically, an NFC-compliant circuit includes an antenna circuit having an antenna coil, and an IC chip. An operating voltage for the IC chip is supplied from a built-in battery. The IC chip includes a memory device and a CPU. The memory device is configured to store a chip-specific ID number and various transmission/receipt data, and the CPU is configured to control transmission and reception of signals and writing and reading of data. A card equipped with such an IC chip is sometimes called a contactless IC card since it is capable of transmitting and receiving data without contact with a reader/writer device located at a short distance.
An IC chip in a contactless IC card is supplied with an operating voltage by output from the antenna circuit. More specifically, the antenna circuit includes an antenna coil and a capacitor resonating therewith and operates efficiently at its resonance frequency. This resonance frequency is set to 13.56 MHz, which is equal to the frequency of a carrier wave transmitted from the reader/writer device. When the contactless IC card is held close to a reader/writer device, the carrier wave is received by the antenna circuit and the power of the received carrier wave is rectified to supply an operation voltage to the IC chip. Thus, no operation voltage is required from a power supply such as a battery to perform processes such as data transmission/reception to and from the reader/writer device. Accordingly, the resonance frequency of the resonance circuit must be properly adjusted to the frequency of the carrier wave to supply a stable operation voltage to the IC chip. Meanwhile, when the IC chip of a contactless IC card is installed on an electronic instrument equipped with a battery such as a mobile phone, the IC chip is supplied with power from the battery of the electronic instrument. In this case, the resonance frequency of the resonance circuit still must be properly adjusted to the frequency of the carrier wave to enhance the sensitivity of the transmission to and reception from the reader/writer device.
However, in actual manufacturing processes, electric characteristics of the antenna coil and the capacitor can be varied from their designed values and circuit elements can be displaced from the right positions, which can cause the resonance frequency to be off the designed value of 13.56 MHz. Accordingly, the resonance frequency must be adjusted to the carrier wave frequency after manufacturing.
There are known techniques for adjusting the resonance frequency after manufacturing. For example, Japanese Patent Application Publication No. 2009-200748 discloses an antenna device for use in a contactless IC card, wherein a part of wires connecting the reactive elements of the resonance circuit is cut with laser to adjust the resonance frequency of the resonance circuit discretely.
Additionally, the Applicant has proposed a resonance circuit that includes a variable capacitance capacitor element configured to vary its capacitance with a bias voltage, thereby to adjust the resonance frequency without any mechanical alteration to the wiring pattern (see Japanese Patent Application No. 2011-073607).
There are various known variable capacitance capacitor elements. For example, Japanese Patent Application Publication Nos. 2011-119482 and 2010-055570 disclose variable capacitance capacitors composed of a plurality of dielectric layers and electrode layers stacked together. Japanese Patent Application Publication No. 2006-303389 discloses a thin film variable capacitance capacitor element wherein electrode layers and dielectric layers are thin films.
In adjusting a resonance frequency by using a variable capacitance capacitor element, a bias voltage corresponding to the desired amount of variation of the resonance frequency is determined and applied, based on the capacitance variation characteristics representing the relationship between a DC bias voltage applied to the variable capacitance capacitor element and a capacitance of the variable capacitance capacitor element (see, for example, FIG. 11(b) of Japanese Patent Application Publication No. 2010-055570). The variation characteristics of the capacitance with respect to the bias voltage are different depending on the materials of the dielectrics constituting the variable capacitance capacitor. To adjust the resonance frequency over a sufficiently wide frequency region, it is preferable to use a dielectric material that provides high variation rate of capacitance with respect to the applied bias voltage.
However, when a DC bias voltage is continuously applied to a variable capacitance capacitor element, the capacitance of the variable capacitance capacitor gradually decreases with time. This phenomenon is called DC aging. In DC aging, the capacitance of the variable capacitance capacitor is shifted from the value determined from the capacitance variation characteristics; as a result, the resonance frequency cannot be adjusted accurately.